Mold is eukaryotic micro-organisms that exists anywhere there is water, warm climate, and a food source.
Mold consumes material like drywall and wood. In our modern day times people generally construct their buildings out of drywall and other wood based products which can result in a number of problems in which mold may arise in the home itself. Mold will grow in damp locations like a basement or a bathroom.
Mold spores are harmful to inhale, especially for people with asthma or who are sensitive to allergies. Thousands of different strains exist but only a handful can be found in any one home.
Mold can be cleaned from the surface, but because mold spores are in the air and on surfaces we can’t see, the mold will grow back. To prevent this from happening, you must eliminate the source of the mold problem. Where did the water damage come from? How is water accumulating on the ceiling? Is the basement flooding?
Once you clean up and resolve the water issue, you need to supply ventilation to those particular rooms and if you can, purify/filter the indoor air or replace your indoor air with fresh outside air. This will reduce the mold greatly and reduce the chances of mold re-appearing. If you are still worried or you still smell the musty odor, you can have a professional test the air for mold.
For more information on mold, visit inspectapedia.com and search for mold. You will discover all kinds of useful sub-topics relating to mold.